Bessie Is this a breakdown in the genetics or just environmental stress from daylength
Mostly a result of daylength but, genetically, some varieties will lose their centers earlier than others (by a few weeks). At my latitude, dahlia blooms are noticeably shallower by the second week in October and most are fully open-centered and unsalable by Nov. 1. For me, this is the best time to hybridize because the centers are open with lots of pollen. I also have a dry November so seeds have a chance to ripen before the rains start.
It is actually the length of night (short dark period) and not the length of day that is important for best bloom formation but for most uses we just say "daylength".. When the daylength at bud formation is shorter than about 12 hours the blooms begin to be much shallower. If dahlias are grown in the shade, or if the days are cloudy, the darkness comes earlier in the day so the decline begins a week or two sooner.
When I grow dahlias year-round I start supplemental lighting the second week of September to make sure the blooms remain fully double. The intensity of light needed is not great (about 10 foot-candles, if I remember). I use the LED equivalent of 100 watt incandescent bulbs placed 5 feet above plants and spaced 10 feet apart. I turn the lights on for about 2 hours, starting at sunset. Just remember, the daylength also influences tuber formation so you might not want to increase the daylength if you want the best tubers.
